The invention relates to a process and an apparatus for the production of iron oxides containing little amounts of residual chlorides originating from free hydrochloric acid containing iron chloride solutions, which are fed into a directly fired reaction chamber heated through burner chambers, yielding iron oxide and regenerated hydrochloric acid by means of spray roasting.
During the production of spray roasted iron oxide by means of thermal decomposition of free hydrochloric acid containing iron chloride solutions originating from steel pickling operations and generally containing between 800 up to 3000 ppm of Cl, it is not possible to retain a specific surface area larger than 3.5 m.sup.2 /g and at the same time reduce the residual chloride ions content of related iron oxide to less than 500 ppm.
For example, K. Suganuma et al. concluded in their publication `Removal of chloride ions from hematite powders for ferrite production` ("Advances in Ceramics" Vol. 15, p. 81-85, The American Ceramic Society Inc., Columbus, Ohio, 1986), that it is possible to lower the chloride ion content by means of several hours long treatments with H.sub.2 O and/or SO.sub.3 at the expense of reducing the specific surface area at the same time. In addition a non-desirable amount of sulfur will be picked up by the iron oxide. Furthermore subsequent rotary kiln treatments with ammonia and with heated screw feeders using steam injection are known.
On an industrial scale this problem is presently solved by employing serveral unit processes like washing with deionized water followed by spray drying and subsequent milling. These additional unit processes are elaborate and cost intensive.
Furthermore another process (WO 96/32355) for the reduction of chloride ions from regenerated iron oxides, originating from the thermal decomposition of hydrochloric acid containing iron chloride solutions, is known. This process proposes subsequent mixing of iron oxide with metal hydroxides along with a thermal treatment in order to reduce chloride ions, but no reference is made in connection with the reduction of the specific surface area.
TDK Electronics Co. in Japanese Patent Application 47-39477 discloses a process for the conversion of beta-FeOOH, containing 2.5 wt. % chloride ions, into alpha-Fe.sub.2 O.sub.3 at 450.degree. C. However no reference is made regarding the possible reduction of chloride ions.
A substantial disadvantage for processing synthetic, regenerated iron oxides originates from the highly corrosive nature of chloride ions even at low concentration levels in respect of construction materials over a wide temperature range Furthermore it is known, that the magnetic and mechanic properties of ferrite materials, which have been produced from synthetic regenerated iron oxides, are drastically influenced by the presence of chloride ions.
Thus, it would be desirable to have a process and device, which produces granules of iron oxides exhibiting a specific surface area in excess of 3.5 m.sup.2 /g and reduces the chloride ion content temporarly, without using excessive unit processing, to less than 500 ppm.
In order to accomplish this task, the present invention provides a process for the production of iron oxides produced from free acid containing iron chloride solutions comprising spraying said solution into a reaction chamber which is directly fired by means of a burner chamber, spray roasting iron oxide and regenerated hydrochloric acid, cooling the spray roasted iron oxide granules down to temperatures of less than 450.degree. C. after passing the burner's focal plane by means of a cooling gas, and discharging said iron oxide granules.
In addition, an apparatus for the production of iron oxides comprising a spray roaster which comprises a spraying device for spraying charged iron chloride solution into a reaction chamber which is directly heated by a burner chamber by which the solution is thermally decomposed into iron oxide granules, HCl gas and combustion gases, an exhaust for the HCl gas containing combustion gases and a discharge device for iron oxide granules, and a cooling zone between the burner chamber's focal plane and the iron oxide discharging device for spray roasted iron oxide granules is provided.